Opportunity Concretion Questions

[munkin]

I recently read a very interesting concretion post on the Bad Astronomy Board. Essentially the questions raised were why the form of the concretions were exactly the same size and form through all the strata in Endurance even though the layers differed chemically.<br /><br />This has become a very vexing question and has left me to puzzle this over the last few months until I read that someone else has questions about this as well. Interestingly, no other posters have brought this up. Should the concretions that are all over Meridiani be in different sizes and shapes according to the chemical layers that they are formed? Would the shapes and sizes tend to change between layers of evaporites as they are chemically different ie. chlorine increases as the layers deepen in Endurance. Why are they all the same size and shape at Meridiani even though most of the concretions on earth are essetially speriodal and differ in size.<br /><br />For example: <br /><br /><br />http://www.desertusa.com/mag98/oct/papr/geo_conc.html <br /><br />http://www.amonline.net.au/factsheets/geodes.htm<br /><br />http://epsc.wustl.edu/admin/resources/meteorites/meteorwrongs/concretions.htm<br /><br />http://home.att.net/~amcimages/hyde.html (please note that these were the only concretion examples that I found completely uniform, had stems and were biological in origin- these were shaped in crab holes)

 

[alpha_taur1]

Jon Clark will hopefully jump in here and explain in more depth, but I think it's important to realise that not everything happened at once. The geochemistry leads us to believe that there were distinct stages in syn-depositional or post depositional alteration. One of the characteristic features is the presence of pseudomorphs, probably after gypsum. So if we look at possible sequences of events, we have the initial deposition, which is probably a mixture of aeolian and possibly shallow lake (or even glacial varve) type sequences. As the water concentrates, the calcium sulfate (Gypsum) crystallises out to form these tabular crystals in the sediment or groundwater soaked strata. There is clearly an acidic stage in which sulfuric acid breaks down some of the Olivine to form possibly Jarosite plus some Epsomite. Changing conditions, perhaps pH variation with concentrating water, leads to the formation of Goethite, which tends to nucleate on to some impurity, forming concretions. Later, as water levels drop, there is local concentration of first sulfates, then chlorides and more soluble bromides, leading to changing concentrations in the layers. <br /><br />That's not necessarily exactly the way things happened - we need more detailed studies for that. But it gives a feel for the fact that it was not just a one stage evaporation, and the spherules perhaps formed at an earlier stage than the evaporate deposition.<br /><br />I hope others will expand on this. It's a starting point, and I'd be happy to develop it into a better model.<br /><br /><br /><br />

 

[blairf]

Exactly this question has led to several threads with hundreds and hundreds of posts over in mark carey's www.markcarey.com/mars forums. Some of the posters are a bit 'out there' but there are some very serious involved posts.

 

[alpha_taur1]

Yes, I used to post over there under the user name of Aldebaran. I agree that it's getting way out there but there are still some serious posters braving the noise.

 

[munkin]

"the spherules perhaps formed at an earlier stage than the evaporate deposition"<br /><br />How can this be unless all the strata were "mixed" uniformly. I understand completely the stages that you mentioned in your post. Groundwater should have percolated upward or surface water downward and concretions formed from within the voids of the strata. What I don't understand is the uniformity of the concretion size and shape. It would be really interesting to hear this explained from a geochemist point of view. Also, most concretions on Earth are formed from a biological nucleus - this trigger has not been mentioned in the possible formation of the Meridiani concretions - at least not from the JPL scientists. I wonder why they have left out that fact?

 

[alpha_taur1]

What I mean is that the evaporate minerals were the last to crystallise out. We could have had a water saturated basaltic sand type deposit, altered somewhat by acid, and containing concretions from the earlier episode (perhaps a period of high volcanic emissions). <br /><br />The evaporates only start to form as the water starts to evaporate. The evaporates effectively alter the existing strata, which is relatively permeable. The evaporation causes salts to form within the strata, and post depositional diagenic processes result in the modification of these strata at various stages.<br /><br />Diagenic processes are the mechanisms by which solid minerals can be formed from dissolved salts. (without the action of living organisms) <br /><br /><br />So that's what I mean when I say that the evaporates formed later.<br /><br />I am not providing any mechanism which accounts for why the water disappeared. <br /><br />Concretions on Earth can also be formed without a biological nucleus. We can go in circles talking about this particular aspect. The fact that the earth is infested with life makes it more likely that biological processes are involved. (In fact many concretions on Earth are predominately biological.)<br /><br />I also find it interesting how the concretions are uniform in size. Concretion growth is not always well understood, even on Earth, and size and distribution can vary enormously between deposits.

 

[centsworth_II]

<font color="yellow">"...why the form of the concretions were exactly the same size and form through all the strata in Endurance even though the layers differed chemically."</font><br /><br />My own simple way of seeing it is that the layers formed without concretions. After all the layers formed, they were soaked with water of a particular chemical nature that led to concretion formation. So there was a concretion formation episode which took place in all layers at the same time <b>after</b> they were all laid down. In other words, its not the chemical nature of the individual layers that ruled concretion formation, but the chemical nature of the water moving through all the layers.<br /><br />I have not followed the in depth concretion discussions so I may be off base. <div class="Discussion_UserSignature"> </div>

 

[munkin]

Interesting point - but there must have been a very *deep* layer of acidic water bearing a substantial amount of dissolved iron to permeate the layers of strata and create such a huge amount of hematite. It will be very interesting to find out how deep the concretions actually go below the surface and how high the above salt layer was before weathering left the concretion layer on the surface. <br /><br />The sea above Meridiani must have been very deep, stable for a long period of time (and evaporated rapidly) or if shallow, persisted for 10's of millions of years to accumulate the amount of iron needed to form the concretions along with the salt evaporites. <br /><br />

 

[JonClarke]

You rang, Alpha Tauri?<br /><br />Munkin, it is important to realize that geological reasoning is inferential and sifts multiple working hypotheses. It therefore tends to be long-winded also! So apologies for the length of the following.<br /><br />The Martian spheroids, when first observed, could have been impact spherules, concretions, ooids, accretionary lapelli, weathering pisoliths, even fossils. Careful observations led to the development of the concretion hypothesis. <br /><br />Their mineral (haematite-rich) and chemical (iron oxide composition, is consistent with them being concretions pisoliths, ooids and fossils. The presence of grains growing together to form doublets and triplets also argued against them being accretionary lapelli or impact spherules, but is consistent with them being ooids, concretions, or pisoliths. The lack of high levels of repeatability and organization argues against them being fossils, and the absence of independent evidence of life Mars makes the explanation problematic as a stand-alone hypothesis. The way the features do not form lags in the rock and displace the sediments shows they have grown in place, ruling against ooids but is consistent with concretions and pisoliths. The fact that they do not appear to distributed in a weathering profile argues against them being related to weathering but are more likely due to burial diagenesis, as does the relatively even distribution throughout the rock and the absence of recent near-surface water. Thus concretions are the most likely explanation. Pisoliths are the second choice. We have good terrestrial counterparts to both. The presence of haematite fill in fractures supports iron migration in groundwater.<br /><br />With respect to your specific questions, first the concretions do vary in size. While this size range is tightly constrained, some terrestrial concretions show similar size-frequency distributions, as do pisoliths. <br /><br />Second, although they superficially loo <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[JonClarke]

Well, that certainly killed the conversation! <img src="/images/icons/smile.gif" /><br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[alpha_taur1]

Jon, <br /><br />Thanks for responding. I hoped that you would. I'm out bush on a dialup, and I'll take a closer look this weekend.

 

[alpha_taur1]

"The sea above Meridiani must have been very deep, stable for a long period of time (and evaporated rapidly) or if shallow, persisted for 10's of millions of years to accumulate the amount of iron needed to form the concretions along with the salt evaporites. "<br /><br /><br />I think that the most we can say is that there was an awful lot of water <img src="/images/icons/smile.gif" /><br /><br />There is plenty of iron in the basaltic sand, in the form of Olivine, an iron(II) magnesium silicate. There must have been a considerable thickness of sediment, which without appreciable concentrations of clay minerals, must have been quite permeable.<br /><br />I agree, it will be interesting to see how deep the concretions go, and perhaps we'll find out if Opportunity makes it to the etched terrain.

 

[munkin]

A belated thank you - Jon, for your very informative post. <br /><br />How I do wish those rovers were equipped to measure not only chemical compositions - but isotopes as well. Hopefully, one of the future rovers . . . <br /><br />One more quick question - you stated:<br /><br />"Second, although they superficially look very similar, closer examination shows that the range in shape from highly spherical to quite irregular, and includes doublets and triplets. This is exactly as you would expect for concretions."<br /><br />The irregular concretions are usually scattered about the plain or have already weathered out of the sedimentary matrix. I tend to believe that thier shapes are irregular due to mechanical weathering - perhaps even chemical as we have seen evidence of frosts. <br /><br />I have yet to see one of the concretions freshly exposed on the surface (or beginning to emerge from the rock due to weathering) in any shape other than highly speriodial. Some even appear to be perfectly round. They also do appear to somewhat self limit their size. I am wondering if there is any way to recreate these in a lab setting. <br /><br />

 

[munkin]

I actually spent a few hours over there after I viewed your post. A few posts were very interesting but you are right some were way out there. . . <br /><br />Thanks again for the link.

 

[JonClarke]

As I understand it Phoenix, MSL and ExoMars will all be able to do isotopes. There is such a thing as mission constraints, short of a massive mission like ExoMars or MSL, you have to chose. The Rovers certaibnly do more than parthfinder in terms of analyses.<br /><br />Yoyur point about degradation is interesting. It is important to note that the lag allows us to look at a lot more concretions that we might otherwise see. But the lag grains do seem to be fairly resistant. The concretions do break, there are halves visible in some of the images. But the doublets and triplets look primary to me and certainly do not seem eroded. The really irregular ones might be fragments of the veins.<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[alpha_taur1]

Actually I agree that some of the surface spherules on the plain appear to be highly eroded. I guess it stands to reason.<br /><br />Jon, would you be so kind as to explain the difference between a pisolith and a concretion. <br /><br />I think you'd do it more elegantly than I would.<br /><br />- Jim

 

[JonClarke]

Hi Jim<br /><br />We'll have to take a dive into the muddy waters of arcane geological terminology and the rather loose way it is used by many geologists - even me! <img src="/images/icons/smile.gif" /><br /><br />Strictly speaking a pisolith (somtimes pisoid or psiolite, although the latter means a rock made of pisoliths) is a concrentric rounded grain formed at or near the surface that is larger than 2 mm. It is commonly restricted (as I used it) to grains formed throughg weathering, although the term as been applied to cave pearls, large ooids, and spheroidal hydrothermal precipitates. So the ironstone gravel that covers much of Australia common contains psioliths, Bauxites such as Weipa are often pisolithic.<br /><br />A concretion is a lump of mineral of any size, shape, and composition that grows in situ in an indurated rock through diagenesis or weathering.<br /><br />The spherules at Meridiani could be called pisioliths with some justification but are called concretions I suspect to avoid an unwarrented link to formation at or near the surface.<br /><br />Hope this helps.<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[alpha_taur1]

Thanks for that explanation, Jon.<br /><br />Interesting, but when I first saw the Meridiani spherules, I thought immediately of the lateritic pea gravel that can be found in many parts of Western Australia, particularly the hills around Perth where they have been responsible for many a slip and fall.

 

[JonClarke]

Hi Jim<br /><br />Although the grains are somewhat different in origin, I think the surface lags over the Yilgarn in many was is analogous to what we see at Merdiani. However, some of the palaeovalley fills near Kalgoorlie contain beautifully spherical concentric concretiosn up to 1 cm across in acid-weathered fine-grained sediments that look eerily like those at Meridiani. A colleague of mine was other there a few weeks ago collecting samples, hopefully we can get enough to work on. While we are studying these for another reason, i hope to develop a mars analogue aspect to them.<br /><br />Munkin, I missed your earlier comment about experiments and self limiting. I don't think anyone has successfully dulpicated the form of concretions in the lab, it is probably because of the time scale needed. However the chemical reactions that precipiate them are quite easy to duplicate. I agree that there some kind of self limiting going on with them, which is why we don't see them larger than cm size.<br /><br />Cheers<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>